本文選題：P91耐熱鋼 + δ-鐵素體�。� 參考：《蘭州理工大學(xué)》2017年碩士論文

【摘要】：為了能使電力發(fā)展和環(huán)境保護(hù)同時(shí)得到共存和發(fā)展,發(fā)展大容量發(fā)電機(jī)組刻不容緩,關(guān)鍵是耐熱材料是否滿足電力行業(yè)鍋爐所需承受的高溫和高壓。P91耐熱鋼的出現(xiàn)解決了這一問題,其優(yōu)良的性能使得電力行業(yè)的發(fā)展產(chǎn)生了質(zhì)的飛躍。但P91耐熱鋼在熱加工(熱處理、焊接)過程中可能出現(xiàn)的δ-鐵素體組織會(huì)影響材料的性能。本文研究了直接升溫、不同熱處理工藝和不同TIG工藝條件下,P91耐熱鋼中δ-鐵素體轉(zhuǎn)變、形態(tài)和體積分?jǐn)?shù),及其對(duì)材料性能的影響。為工程實(shí)際應(yīng)用中解決P91耐熱鋼中δ-鐵素體問題提供了理論基礎(chǔ),也對(duì)熱處理工藝和焊接工藝的優(yōu)化提供了借鑒。首先利用激光共聚焦顯微鏡原位觀察鋼P(yáng)91升溫過程中的固態(tài)相變,直接觀測(cè)得出升溫過程中鐵素體(α)到奧氏體(γ)以及奧氏體(γ)到高溫鐵素體(δ)的金相組織轉(zhuǎn)變過程以及相變溫度,觀察δ-鐵素體在升溫和降溫中組織的轉(zhuǎn)變,為后面熱處理工藝參數(shù)提供借鑒。其次,通過制定熱處理工藝來研究不同熱處理溫度下獲得的δ-鐵素體組織,主要分析了不同正火溫度對(duì)P91鋼中δ-鐵素體含量的影響,試樣顯微組織經(jīng)Vilella試劑腐蝕顯示在金相顯微鏡(OM)中呈現(xiàn)亮白色,并且呈現(xiàn)塊狀多邊形形貌,而隨著正火溫度的升高δ-鐵素體體積分?jǐn)?shù)增加,其中δ-鐵素體中的鐵素體形成元素Cr、Si含量稍高于馬氏體基體,奧氏體形成元素Mn稍低于馬氏體,試樣性能測(cè)試中發(fā)現(xiàn)δ-鐵素體對(duì)P91韌性影響很大,隨著δ-鐵素體體積分?jǐn)?shù)的增加P91沖擊韌性下降,斷口形貌從韌脆混合斷裂轉(zhuǎn)變?yōu)榇嘈詳嗔?δ-鐵素體組織的出現(xiàn)造成P91鋼的選擇性腐蝕,隨著δ-鐵素體體積分?jǐn)?shù)增加,P91鋼的耐腐蝕性降低。最后,通過焊接工藝來研究P91鋼中的δ-鐵素體,以TIG焊為焊接熱源來改變焊接過程中的熱輸入,結(jié)果表明,隨著熱輸入的增加δ-鐵素體含量先增多隨后趨于穩(wěn)定,δ-鐵素體分布不均勻,呈多邊形分布于熱影響區(qū),以及少量呈條狀分布于焊縫,顯微維氏硬度計(jì)下亮白色多邊形δ-鐵素體顯微硬度均值為235HV0.2,馬氏體均值為373HV0.2,δ-鐵素體含量對(duì)接頭沖擊韌性也有顯著影響,隨著δ-鐵素體含量增多,接頭的沖擊韌性下降,沖擊斷口形貌從韌/脆混合斷裂轉(zhuǎn)變?yōu)榇嘈詳嗔?從斷口縱截面處的裂紋擴(kuò)展形貌可見裂紋主要在馬氏體和δ-鐵素體的界面處產(chǎn)生,并沿多邊形δ-鐵素體一邊向材料內(nèi)部擴(kuò)展。
[Abstract]:In order to make electric power development and environmental protection co-exist and develop simultaneously, it is urgent to develop large capacity generator sets.The key is whether the heat resistant material can meet the high temperature and high pressure. P91 heat resistant steel, which is required by boiler in power industry, solves this problem, and its excellent performance makes the development of power industry make a qualitative leap.However, the 未-ferrite structure may affect the properties of P91 heat resistant steel during hot working (heat treatment, welding).In this paper, the transformation of 未 -ferrite, morphology and volume fraction of 未 -ferrite in P91 heat-resistant steel under direct heating, different heat treatment processes and different TIG processes were studied, and their effects on the properties of the steel were studied.It provides a theoretical basis for solving the 未 -ferrite problem in P91 heat-resistant steel in practical application, and also provides a reference for optimization of heat treatment process and welding process.First, laser confocal microscope was used to observe the solid phase transition during the heating process of steel P91 in situ.The microstructure transformation process from ferrite (偽) to austenite (緯) and austenite (緯) to high temperature ferrite (未) was observed directly, and the microstructure transformation of 未 -ferrite during heating and cooling was observed.It provides reference for the process parameters of heat treatment.Secondly, the effect of normalizing temperature on the content of 未 -ferrite in P91 steel was analyzed by making the heat treatment process to study the 未 -ferrite structure obtained at different heat treatment temperature, and the effect of normalizing temperature on the content of 未 -ferrite in P91 steel was analyzed.The microstructure of the sample was shown to be bright white in the metallographic microscope by Vilella reagent corrosion, and the morphology of the bulk polygon was observed. However, with the increase of normalizing temperature, the volume fraction of 未 -ferrite increased, and the volume fraction of 未 -ferrite increased with the increase of normalizing temperature.The Cr-Si content of ferrite forming element in 未 -ferrite is slightly higher than that of martensite matrix, and mn is slightly lower than martensite. It is found that 未 -ferrite has a great influence on the toughness of P91.With the increase of volume fraction of 未 -ferrite, the impact toughness of P91 decreases, the fracture morphology changes from ductile-brittle mixed fracture to brittle fracture, and the appearance of 未 -ferrite structure results in selective corrosion of P91 steel.The corrosion resistance of P91 steel decreases with increasing the volume fraction of 未-ferrite.Finally, the 未 -ferrite in P91 steel is studied by welding process. The heat input in the welding process is changed by TIG welding. The results show that,With the increase of heat input, the content of 未 -ferrite first increases and then tends to be stable. 未 -ferrite is distributed inhomogeneously in the heat affected zone, and a small amount of 未 -ferrite is distributed in the weld seam.The average microhardness of bright white polygon 未 -ferrite is 235HV0.2, and the average of martensite is 373HV0.2. The 未 -ferrite content also has a significant effect on the impact toughness of the joint. With the increase of 未 -ferrite content, the impact toughness of the joint decreases.The fracture morphology of impact fracture changed from ductile / brittle mixed fracture to brittle fracture. From the crack growth morphology of longitudinal section of fracture surface, the cracks mainly occurred at the interface of martensite and 未 -ferrite, and propagated to the interior of material along the polygonal 未 -ferrite side.
【學(xué)位授予單位】：蘭州理工大學(xué)
【學(xué)位級(jí)別】：碩士
【學(xué)位授予年份】：2017
【分類號(hào)】：TG142.1;TG161;TG457.11

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